Perigenual anterior cingulate cortex (pACC): also part of the limbic system, associated with emotional regulation, motivation, and schizophrenia

Dorsolateral prefrontal cortex (DLPFC): part of the cognitive center of the brain, associated with working memory, planning, reason, and inhibition

Structural equation modelling (SEM) was used to determine the relative amount that each of the 4 land-use types influenced changes in size and activity of the measured brain regions.

Key Findings

Of the 4 land-use types tested, only residential proximity to a forest had a measureable effect on the 3 brain structures, and of these only the amygdala showed statistical significance (β=0.232, SE=0.090; P=0.010). This result was valid at the 1 km radius, and remained significant when retested at 500 m and 2 km radii.

Practice Implications

By now it should be understood that exposure to natural settings can benefit mental health, including mood, attention, and memory1 as well as rates of morbidity and mortality.2 This current study demonstrates that key anatomical changes may be occurring from prolonged environmental exposures. In particular, the association here between forests and changes in amygdala activity suggest that living in sylvan places does not just promote an acute sense of relaxation during exposures, but may facilitate increased long-term resiliency against negative emotions such as fear and anger by reshaping the brain.

This is not surprising, as the neuroplastic capacity of the brain has been recognized for many decades.

This study continues the pioneering work of Lederbogen and colleagues, who demonstrated that an adult’s limbic system may be influenced by the greenery surrounding their childhood home.3 Essentially, a more rural childhood can benefit how a person’s brain (specifically the amygdala and pACC) processes and perceives stress as an adult, many years after they have relocated. This is similar to research showing that an activity such as meditation training can have beneficial effects on emotional reactivity and corresponding limbic system activity that persist years after the initial meditation training.4 In this sense, where you live might be just as important for developing successful shifts in stress coping capacity as what activities you do.

This is not surprising, as the neuroplastic capacity of the brain has been recognized for many decades.5 Studies with rats have been conducted since the late 1940s demonstrating the impact of “environmental enrichment” on both brain structures and corresponding behavior, mood, and memory.6,7 The importance of suitably enriched environments is standard practice in zoological settings with noted impacts on animal brain structure and function, as well as morbidity and mortality.8 It makes sense that human beings would also be affected by the environmental context in which they live.

Increasingly for humans, this context is the urban environment. According to the 2010 US Census, over 80% of the US population lives in an urban area, with projections for continued urban growth.9 This is very different than the environmental context in which human beings evolved and toward which we are optimally adapted, according to concepts such as EO Wilson’s Biophilia Hypothesis10 or Ulrich’s Psycho-Evolutionary Stress Theory.11 The urban environment produces many stressors, including traffic, noise pollution, air pollution, and population density, all of which have measureable detrimental effects on mental health and well-being.12 The psychophysiological impact of urban living is known as “urban stress,” a concept coined in 197713 and well-researched since that time.14,15 A meta-analysis of 20 studies conducted in 2010 revealed a 13% to 28% increased odds ratio of mental disorders, including depression and anxiety, for urban vs rural residents.16 Some research has shown that rates of schizophrenia are up to 2.5 times higher in urban vs rural environments, even after adjusting for reasons why people with the condition might preferentially relocate to cities (eg, greater access to mental health services).17

While there is more work to be done to understand the complex interaction of environmental and individual factors involved, it is clear that our surroundings are more than simply a passive part of our experience. They are an integral component of what determines the health of our minds and bodies.

Limitations

This was a cohort study of individuals living in residences of their choosing, not an experimental design. Therefore, it is not possible to attribute causality to land-use type based on these results. However, this study design is very common in public health research, as it would be unethical, and very costly, to randomly assign different residential locations and require participants live there for multiple years.

Another limitation of the study is the 9-year gap between the land-use dataset (2006) and the brain scan data (2015). While land-use type in Berlin does not change very rapidly, it is possible that the 2015 land-use data would be different and thus could affect the data outcomes.

Lastly, this study only looked at 1 km radii of participants’ residential addresses, not where they spent their time or what they looked at. It is possible that their environmental exposures were somewhat different from what is assumed by this study. For example, the study did not measure how much time participants spent outside their homes or looking out their windows, or how much time was spent at these locations. However, given the mean age of participants (70 years), it is likely that significant amounts of time were spent residing at this location.

Conclusions

This study adds to the research evidence that environmental factors influence structural changes in stress-related areas of the brain. Physicians may choose to incorporate this information into aspects of their clinical encounters relating to either patient history–taking (eg, “How green is it where you live?”) or recommendations for therapeutic benefit (eg, prescriptions for increased green space exposures).

About the Author

Kurt Beil, ND, LAc, MPH is a naturopathic and Chinese medicine practitioner in New York’s Hudson Valley region, as well as adjunct faculty at the National University of Natural Medicine (NUNM) and the Academy of Integrative Health & Medicine (AIHM). He completed his postdoctoral research at NUNM’s Helfgott Research Institute, where he focused on biomarker and psychometric assessment of the restorative and therapeutic effect of natural environments. He is the founding co-chair of the Health & Nature subcommittee of the Intertwine Alliance, a 150+ member coalition of nonprofits, governmental agencies, and private businesses promoting the parks, trails, and natural areas of the Portland Metro region. Dr Beil speaks frequently on the health benefits of contact with nature, and maintains a Facebook group (“Naturopaths for Nature”) about this topic. He can be reached at drkurt@earthlink.net or www.hudsonvalleynaturalhealth.com.